This invention relates generally to semiconductor devices, particularly to field-effect semiconductor devices as typified by high electron mobility transistors (HEMTs), and more particularly to such devices that are normally off. The invention also pertains to a method of making such normally-off field-effect semiconductor devices.
The HEMT of typical prior art design comprises an electron transit layer of undoped gallium nitride (GaN) grown on a substrate of silicon, sapphire or the like via a buffer, an electron supply, or barrier, layer of n-doped or undoped aluminum gallium nitride (AlGaN) on the electron transit layer, and a source, drain, and gate (Schottkey) electrode on the electron supply layer. The AlGaN electron supply layer has a greater bandgap and less lattice constant than does the GaN electron transit layer.
Overlying the electron transit layer of a greater lattice constant, the electron supply layer experiences an expansive strain (tensile stress) and so gives rise to piezoelectric depolarization. The AlGaN electron supply layer is also subject to spontaneous depolarization. Consequently, what is known as a two-dimensional electron gas layer is created in the electron transit layer adjacent its heterojunction with the electron supply layer. The two-dimensional electron gas layer provides a path, usually referred to as a channel, of current flow between the source and drain electrodes. The source-drain current flow is controllable by a bias voltage impressed to the gate electrode.
The HEMT of the foregoing general prior art construction was normally on, there having been a source-drain current flow while no voltage was being applied to the gate electrode. It had to be turned off using a negative power supply for causing the gate electrode to gain a negative potential. Use of such a negative power supply made the associated electric circuitry unnecessary complex and expensive. The conventional normally-on HEMT was therefore rather inconvenient of use.
Attempts have been made to devise a HEMT that is normally off. One known approach to that end was to make the AlGaN electron supply layer thinner. A thinner electron supply layer weakens the field of the electron supply layer due to piezoelectric and spontaneous depolarizations, resulting in the diminution of electron concentration in the two-dimensional electron gas layer. The two-dimensional electron gas layer disappears at its part just under the gate when a field due to the potential difference, (built-in potential) between the electron supply layer and, making Schottky contact therewith, the gate electrode acts upon the two-dimensional electron gas layer of reduced electron concentration. The HEMT can thus be held off between the source and drain electrodes without application of a bias voltage to the gate.
However, the normally-off HEMT based upon this conventional scheme proved to possess the drawback that, by reason of the thin electron supply layer itself, the two-dimensional electron supply layer suffers an unnecessary drop in electron concentration at other than right below the gate as well. The result was an inconveniently high source-drain turn-on resistance.
A solution to this inconvenience is found in Japanese Unexamined Patent Publication No. 2005-183733. It teaches to make the electron supply layer thinner only at its part underlying the gate by creating a recess in that layer. This solution proved unsatisfactory in that the creation of the recess by selective etching of the electron supply layer might lead to the impairment of the crystalline structure of the electron supply layer, as well as that of the electron transit layer, and hence to the deterioration of the electrical characteristics of the HEMT. What is worse, in desired mass production of the normally-off HEMTs, their threshold voltage would fluctuate from one device to another if, as is very likely to occur, their electron supply layers were not etched to an exactly unvarying depth. For these reasons, as far as the applicant is aware, there seem to be no normally-off HEMTs of the above known scheme that have been currently available on the market.
Japanese Examined Patent Publication No. 8-264760 makes a different approach to a normally-off HEMT. It suggests to make that part of the electron supply layer which underlies the gate electrode higher in resistance by ion implantation. The resulting device is normally off as the heterojunction between the electron supply layer and electron transit layer normally disappears from under the gate electrode. The device turns on upon voltage application to the gate electrode because then the channel is completed by virtue of the field effect through the electron transit layer including its part under the gate electrode. This prior art device is also objectionable by reason of difficulties in invariably creating the high resistance part of the electron supply layer under the gate electrode.